1. Field of the Invention
This invention relates to semiconductor laser devices with a plurality of light emitting layers having different band gaps and methods for driving the same which are to be utilized in wavelength or frequency division multiplexing optical communication systems, wavelength division multiplexing optical recording systems, optocal operation systems and the like and, more particularly, to semiconductor laser devices for emitting laser lights of different wavelengths by controlling the magnitude of a current injected thereinto and driving methods therefor.
2. Related Background Art
In recent years, demands for semiconductor laser devices have rapidly been increasing in fields of optical communications and optical information processings, and accompanied therewith requirements for functions of devices to be used in such fields have become diverse. Among them, there exist tunable (wavelength of a radiated light is changeable) semiconductor laser devices. For example, in a case where a laser light is applied to a record medium such as an optical card and an optical disc for performing information recording and reproduction, information writing by a reproducing light is normally prevented by reducing the magnitude of the reproducing light to a value less than that of a recording light for recording information on the record medium. In this case, if a tunable semiconductor laser is used and a wavelength of its reproducing light is set to be within a range in which a light sensitivity of a medium is low, the prevention of information writing by the reproducing light is effected without reducing the magnitude thereof so much. Thus, the reproduction of information can be performed with a high S/N ratio by the reproducing light having a sufficient magnitude.
In order to satisfy the above-mentioned requirement, there has been developed a first prior art tunable laser device in which light emitting layers for selectively emitting lights of different wavelengths are respectively formed in different light waveguides formed on a common substrate and a laser oscillation is conducted in the light emitting layer of a desired oscillation wavelength by a current injection thereinto (see Appl. Phys. Lett. vol. 36, p. 442(1980)). In this case, it can be said that essentially separate or independent laser devices are formed on a common substrate.
The first prior art device, however, has the following disadvantages.
When the emission or oscillation wavelength is changed, the location of light emission from the device is varied. As a result, for example, where an external optical system for condensing a radiated light from the laser device is structured so that the light of a certain wavelength is condensed into a point, a focused position will be shifted far greater than a small shift due to a wavelength dispersion caused when the emission wavelength is changed. Further, since there is a need to form separate, independent laser devices on a common substrate and to drive them separately, a fabrication process will be complicated and a size of the device will be large.
Next, there has been developed a second prior art tunable laser device which is a distributed Bragg reflector (DBR) type semiconductor laser using a grating as a reflector. In the device, an electrode is further provided in the grating portion for injecting carriers therethrough, and an amount of injection current thereinto is increased or reduced so that the oscillation wavelength is changed by varying a refractive index of the grating portion. In this case, structures of light emitting layers, etc., are the same as those of a normal semiconductor laser.
The second prior art device, however, has a drawback as follows. A range of variable wavelength is narrow, and in a case where, for example, Al.sub.x Ga.sub.1-x As is used, such range is several nanometers. This is because the change of Bragg wavelength by the control of an amount of current injection which has such length of range decides the width or range of variable wavelength.
There has further been developed a third prior art tunable laser device in which a single quantum well layer constitutes of a light emitting layer and a light emission from a quantum energy level higher than a first or ground one is made possible by increasing loss of its resonator. Laser oscillations of different wavelengths are obtained from light emissions from the first and second quantum levels.
FIG. 1A shows an energy band of such single quantum well layer 101 and layers adjacent thereto. FIG. 1B shows its gain spectrum. In a laser having a conventional normal resonator loss, an oscillation threshold gain is g.sub.tho, and its gain spectrum has a peak at a wavelength .lambda..sub.1 corresponding to an energy gap E.sub.g11 for the first quantum energy level at which the laser oscillation is performed. In the third prior art device, its resonator loss is increased, and its oscillation threshold gain is g.sub.tho '. Thus, the laser oscillation of a wavelength .lambda..sub.2 corresponding to an energy gap E.sub.g12 for the second quantum level is made possible.
In the third prior art device, however, the laser efficiency becomes small because the resonator loss is increased for achieving its object. As a result, the oscillation threshold current should be made large and its output cannot be made large. Therefore, a continuous emission at room temperature could not be obtained in such device which is to be act as a two-wavelength tunable laser.
Moreover, there has been disclosed a fourth prior art device in Japanese patent pre-examined publication No. 63-32982. In the fourth prior art device, there are provided two different quantum well layers respectively functioning as light emitting layers for emitting lights of slightly different wavelengths and laser oscillations of different wavelengths are achieved by the respective quantum well layers.
The fourth prior art device, however, also has the drawback that two oscillation wavelengths are quite close to each other or that its range of variable wavelength is narrow.